Advice Centre by Photography Experts (Part 6) - Effective AF points

6/13/2012 9:25:44 AM

Effective AF points

Why do AF points only work towards the centre of the sensor in a DSLR and is this likely to change in the future?

Description: Effective AF points

However a manually assignable AF point can be moved around

Autofocus systems work by detecting contrast differences between adjacent pixels in the sensor. The higher the contrast difference between the pixels, the more accurate the focusing will be.

All the AF points in a digital SLR should work effectively, though the central point is always the most sensitive. The number and distribution of AF points varies from model to model. If you have an entry-level digital SLR then it may have a relatively low number of AF points and those points will generally be towards the centre of the sensor. However, high-end DSLRs have dozens of AF points (Canon's EOS 5D Mk lllin has 61!) covering most of the sensor area so that subjects off-centre can still be picked-up by the AF system. AF points are clustered more towards the centre of the image area simply because that's where we tend to place our main subject. It's unlikely you're going to put an important part of a shot at the very edges of the frame, so there's no need to have AF points there. That said, as the number of AF points in DSLRs increases, you may soon find that they do progress to cover the entire image area.

I'm off travelling soon and intend to upload images to my website, Flickr and Facebook as I go. If I photograph local people, do I need to get them to sign a model release form?

Description:  AF point display via the Quick Control menu in Live View

AF point display via the Quick Control menu in Live View

For the uses you're talking about, no - you don't need model release forms. There's no law against putting images of people on websites and photo-sharing sites without their permission, providing you don't use the images in such a way that could be construed as defamation of character - for example, referring to someone as a drunk if you photograph them having a drink. Even if you did, the chance of the person ever finding out and taking action is very slim, but you should respect the people you encounter and photograph on your travels.

If at a later date you decide to submit some of your images to a photo library, you may be asked to provide model releases for all people shots. If you don't have releases you need to declare this. The library may then decide to either reject the images, or apply restrictions to them. Again, there's no law against publishing photographs of people without their permission, providing they are not depicted in a negative way, so most libraries will accept non-released portraits for use in travel guides, magazines, brochures etc. It's only really for advertising use that model releases are required, because you can't use someone's face to endorse a product or service without their permission - and a signed model release gives you that permission.

Another factor to consider is the effect of asking people you encounter to sign what is, in effect, a legally binding contract moments after you've met them! It's quite a big ask and could spoil what would otherwise have been a pleasant and casual experience!

Colour confusion

I am getting confused about what would be the best set-up for me regarding colour space. My Nikon D90 gives me the option of shooting in sRGB or Adobe RGB; I use Adobe RGB. For software, I use Lightroom 3 which I set up to use ProPhoto RGB. I read that ProPhoto RGB encompasses the Adobe RGB range and is slightly larger, so I figured it was best to have the largest range possible. My monitor however is sRGB, so some colours I would not be able to see. I figure that I would rather that than limit myself in terms of colour. I haven't yet entered the world of printing, but that is next on the agenda. Is it better to have the same colour space from start to finish or is there some logic in my logic?!

Description: Colour confusion

Colour confusion

We spoke to Simon Prais, technical director at Color Confidence ( to gain an insight into the different colour space options. The basic answer is that it is preferable not to convert images from one RGB colour space to another during the image editing, only when preparing them for print or web.

Converting from sRGB to the larger Adobe RGB (or Pro Photo RGB) colour space will not improve your colours. The conversion process will in fact negatively affect image quality. You're far better off maximising the use of your current gamut by increasing colour saturation.

Similarly, converting from a large colour space like Adobe RGB to a smaller sRGB will reduce the colour gamut of your image and should only be applied if the required output is sRGB, which it often is for web images.

The more complex question is which colour space should you being using? Working in Adobe RGB is recommended if you want to benefit from saturated colours when it comes to print. Most professional monitors now display the Adobe RGB gamut but even if they didn't, this colour space can still be used so long as you're not too excessive with editing colour saturation on an sRGB screen.

If you don't have a full understanding of colour management and profile conversions, however, I think it's safer to stick with sRGB. Working in sRGB has it advantages, for example, in situations where your images lack strong saturated colours, such as in portrait photography. In this instance, the range outside of sRGB isn't required and the smaller colour space provides a smaller difference between subtle tones than the larger colour spaces.

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